This invention relates to a stereoscopic X-ray device used for magnification stereoscopic radiography.
A stereoscopic image of an object can be obtained by viewing two radiographs on film, a fluourescent plate or an image intensifier with a stereo viewer. Each of the radiographs contains an exposure corresponding to a focal spot on an X-ray target. The focal spots on the X-ray target are separated by a distance. A stereoscopic image is produced because there is a slight difference in the exposures on the two radiographs.
A stereoscopic X-ray tube is a well known device for generating X-ray radiation from two focal spots as described in U.S. Pat. No. 3,250,916 issued to Rogers on May 10, 1966. This tube comprises an evacuated envelope, a cone-shaped X-ray target and a pair of cathodes facing the target. Two electron beams emitted by the cathodes bombard the surface of the target to form two X-ray generating focal spots. These focal spots are separated by a distance on the surface of the target which is equivalent to the interpupillary distance. Radiographs are produced by exposing film to the X-rays alternately generated by different focal spots. The film is positioned in contact with the back of the object. The technique disclosed in the Rogers patent for producing stereoscopic images is called the contact stereo technique.
In order to enhance the stereo effect, the two focal spots on the surface of the target can be separated by more than the interpupillary distance. In practice, the two focal spots should be separted by a distance greater than one tenth the distance between the focal spot and the film. In other words, the two focal spots must be separated by more than 100 mm if the distance between the focal spot and the film is 1 meter. If a separation of 100 mm is desired, a rotating target diameter greater than 100 mm, for example 150 mm, is required for the stereoscopic X-ray tube because of the inclination of the cone-shaped target surface to the cathode. Consequently, to enhance the stereo effect, a large X-ray tube may be necessary. Of course, a large X-ray tube requires a large sized X-ray shield housing which is difficult to handle for radiography.
The enhanced stereo effect for the contact stereo technique discussed above can be obtained by radiographic magnification stereo techniques using two focal spots separated by a relatively small distance. This magnification stereo technique is described in an article entitled "A New Method of Cerebral Angiography", Neuroradiography Vol. 124, Aug. 1977 (Doi et al). This article describes a system which obtains the same depth perception as the contact stereo technique by using two focal spots which are separated by a relatively small distance. The focal spots are smaller than the focal spots in the contact stereo technique. However, the structure of the stereoscopic X-ray tube is not described in this article.